SPECIFIC AIMS Millions of people die or are disabled each year from acute stroke or heart attack, which are caused by the highly localized formation of blood clots resulting in occlusion of the carotid or coronary arteries. In cases where premonitory symptoms or signs of disease indicate the risk of an acute event, a cocktail of various anticoagulants and antiplatelet agents is administered both orally and intravenously to prevent clot progression. Even with the use of aggressive standard-of-care regimens, thrombus formation still proceeds unpredictably. Conversely, severe or fatal bleeding problems can arise with the systemically active anticoagulants in use today. This proposal reflects the belief that safer and more potent agents that exert unique local effects on thrombosis but minimal systemic effects could represent a superior alternative to existing agents for acute use and serve as a bridge to chronic oral therapies. Accordingly, the development of safer and more effective anticoagulants remains both a challenge and an active pursuit for management of the unstable clot-prone atherosclerotic plaques. Our goal is to create a highly potent nanoparticle-based surface active peptidlc inhibitors of thrombin (PPACK/bivalirudin) that remain biologically active at the site of clot formation, yet exhibit minimal system anticoagulant effects a short while after injection so as to limit untoward side effects. Furthermore, we seek to demonstrate a molecular imaging capability to allow early detection and monitoring of the process of thrombus formation and/or dissolution following early therapeutic intervention. We anticipate that the system will find use in acute applications such as prevention of thrombus progression in unstable atherosclerotic plaques, vascular prostheses, indwelling lines, or any other hypercoagulable state that necessitates emergent therapy. We will employ cheap and scalable chemistries in the formulation process, and utilize existing diagnostic modalities (MRI) for demonstration of thrombus detection. The translational potential is apparent because both the quantitative imaging systems and the base nanoparticle formulations already are in clinical trials for carotid artery imaging of angiogenesis. The overall goal of this proposal is to arrive at a stable, safe and effective formulation for a first-in-class nanoparticle based direct thrombin inhibitor with dual antiplatelet functionality whose delivery can be quantified with image-based readouts of the active clotting process. Major initiatives comprise: Focus on a predictable and genetically prone animal model of atherosclerosis, ApoE -/- mice, with and without cholesterol feeding to represent a more stable versus more intiamed prothrombotic substrate, respectively, for testing the inhibitors of thrombosis Extensive safety testing incorporating immunological and wound healing measures Development of multiplexed nanoparticle therapeutics including irreversible (PPACK) and reversible inhibitors of thrombin (bivalirudin) to enhance potency; and testing of various linking strategies for NP Testing the system against current standards of care to examine improved benefit and safety profile, including the role of repeat dosing, and establishment of a clinically relevant dosing regimen